The invention relates to communications systems and methods. In a more specific sense, the invention relates to a radio communications system involving frequency hopping, that is, a system in which the channel frequency of a transmitter in the system is rapidly changed or "hopped" among a number of values which are advantageously pseudo-randomly selected, the channel frequencies of the receiver or receivers in the system being correspondingly hopped, all with the aim of improving the security of transmission and making effective jamming more difficult.
When such a frequency hopping system is operating in a crowded frequency band (such as HF), difficulties can arise when the current channel frequency is hopped into a particular channel which is already occupied to an extent which causes undue interference.